Analysis of the testes of H-Y negative XOSxr mice suggests that the spermatogenesis gene (Spfl acts during the differentiation of the A spermatogonia

H-Y antigen negative XOSxr mice, like their H-Y positive XOSxr" counterparts, have testes; but, in contrast to XOSxr" males, XOSxr" testes almost totally lack meiotic and postmeiotic stages of spermatogenesis. The quantitative analysis of the testes of XOSxr males and their XY±Sxr sibs, described in. the present study, identified two distinct steps in this spermatogenic failure. First, there was a reduction in mitotic activity among T, prospermatogonia, so that approximately half the normal number of T2 prospermatogonia were produced. Second, there was a dramatic decrease in the number of A3 and A4 spermatogonia and no Intermediate or B spermatogonia. These reductions were also largely due to decreased mitotic activity, there being a shortage of A( and A2 spermatogonial divisions and no divisions among A3 or A4 spermatogonia. Mitotic activity among the T2 prospermatogonia and the undifferentiated A spermatogonia was normal. This means that the spermatogonial stem cells, which are a subset of the undifferentiated A spermatogonia, are unaffected in XOSxr mice. Sxr is now known to have arisen by deletion of DNA from Sxr. It is clear from the present findings that a gene (or genes) present in the deleted DNA plays a major role in the survival and proliferation of the differentiating A spermatogonia.